1. Field of the Invention
The present invention relates to image compression and restoring method for binary images that can be used for binary image equipment including printers. More particularly, the present invention relates to a technique to enhance the compression rate of binary image by compressing the image not by pixel but by cell that combines four pixels and compressing each cell in two steps.
2. Description of the Related Art
Recently as data volume for multimedia increases at a rapid pace, users' printing needs become to involve various forms such as complex graphics or contents of the Internet. According this trend of increasing data capacity, development researches are carried out to speed up the printing speed of printers.
Currently the printers with 600 dpi resolution prevail, but a higher resolution is expected for printers as technology develops. As the printing resolution becomes higher, data volume for treatment increases, which has a great impact on the data transmission time between the host and the printer.
The binary image compression techniques have been developed originally for facsimiles, and then later applied for printers. JBIG, which is a standard for binary image compression, has the following features.
1. The resolution is lowered for progressive transmission.
2. The pre-defined template is used for predicting the probability of pixel.
3. The encoder uses the binary arithmetic coding technique.
4. Using lookup tables for arithmetic operations for the probability estimation and compression dispense with multiplications and divisions.
JBIG that has aforementioned features shows an excellent performance for document images produced by document editors or graphic image involving sequential screens. On the other hand JBIG has not shown such an excellent performance in compressing images produced by the error diffusion method.
Although there is another alternative technique that uses wavelet transform, this technique also does not show a good performance for graphic images while it is rather good at document images.